The amount of expendable hardware required to place a satellite in orbit is the primary cost driver for access to space. Specific impulse (Isp) is one of the major factors determining the size of the vehicle required to accelerate a spacecraft to orbital velocity. As shown in graph 100 of FIG. 1, the specific impulse of rocket propulsion is limited to approximately 450 seconds. Air breathing hypersonic propulsion such as scramjets have much higher specific impulses and have been proposed for space transportation, but they have proven to be impractical due to their limited thrust potential.
Because scramjet propulsion has low thrust and must operate within the atmosphere, gravity and drag losses are high. Under these conditions, rocket propulsion is more efficient. Electrical propulsion has even higher Isp (3,000 seconds or more), but has even lower thrust. As a result, electric propulsion cannot be directly employed as an ascent propulsion system. However, the ability to take advantage of the high Isp of electric propulsion systems in order to propel vehicles into orbit or beyond may be beneficial.